Ive heard the 16ga died thanks to the shotgun manufactures themselves. The story goes that because it was such a perfect balance between a 12 and 20 that they would only sell one gun that could do everything instead of 2. They wouldnt make many popular models in 16ga to force people into buying...
If you can find it winchester longbeard xr 3" #5 is my go to shell. You can't beat it for the money. But you need to buy a few different ones and see what your gun likes best.
Still a good idea to shoot both just to be sure. I always do a pattern at 20 and 40 and 60. So you take 6 shells and you can check both loads at 3 ranges and you're gtg.